Many processes and devices have been used in image processing for digital printing. The acceptance of electrostatic graphic print engines by the graphic arts and commercial print market has heretofore been hampered by various problems, such as mid-tone non-uniform appearance, which includes streaks, banding, graininess and mottle. The image noises at various spatial frequencies, namely graininess & mottle, are of particular interests to this application. In particular, the relative amplitudes of the image noises at different frequencies are adjusted to optimize the visual uniformity of a print.
Ideally, it is desired to eliminate the image noise at all frequencies. But current electrostatic printing technology still leaves a significant amount of noises at various frequencies, which leads to quite objectionable graininess and mottle (blotchy) appearance.
Visual masking is a well known phenomenon that some visual effects are less noticeable in the presence of some other more visually dominant features. Some image processing methods are taking advantages of this phenomenon by using noises to mask out moire, banding and contouring. Another particular example is the visual masking of mottle by graininess: image mottle (low frequency variation) perception improves as the image graininess increases. Therefore, there is an opportunity to optimize the overall uniformity (combining both mottle & graininess) by adjusting the levels of noises at different frequencies. Although it is difficult to lower the system noise, it is possible to increase the noise levels at pre-selected frequencies to achieve the desired visual optimization.
Some image processing methods have been used based on this technology, in which one method injects noise in halftone threshold array rather than directly into the image. The method operates by creating a halftone threshold array of suitable size to support the desired noise characteristics. A texture pattern of corresponding size is created having the desired noise characteristics and boundary transitions. The texture pattern is applied to the halftone threshold array by an amount determined by a noise amplitude control curve. The incoming contone image is then processed with the modified halftone image.
A typical approach in the past has been for introducing certain amount of high frequency grainy noise present on a print to improve the uniformity appearance of prints by partially masking out low frequency noises such as mottle, however, the optimal amplitude of the grainy noise needed on a print depends on the amplitude of the mottle noise. The higher the mottle noise, the greater amount of grainy noise is required. Depending on the system (machine & material) states, some system prints can show a big range of mottle performances (noise level can vary by more than 70%). Therefore, the grainy noise that is optimal for one state is either too much or too little for other machine states and a compromise must be made.
Therefore, as discussed above, there exists a need for an arrangement and methodology which will improve the adaptivity of the noise injection methods through halftone threshold array and other type of image processing. Thus, it would be desirable to solve this and other deficiencies and disadvantages as discussed above, with an improved feedback and feed forward system in which noise injection is being incorporated into the final print to optimize its uniformity appearance by introducing noise injection into the input digital image.